A research study finds that a group of participants who received relaxation training had lower exam anxiety scores than a group who did not receive training. However, the researcher suspects that the difference between groups may be caused by the fact that the participants in one group are generally smarter than those in the other. This study has a problem with

Internal validity

Explanation:

A research study finds that a group of participants who received relaxation training had lower exam anxiety scores than a group who did not receive training. However, the researcher suspects that the difference between groups may be caused by the fact that the participants in one group are generally smarter than those in the other. This study has a problem with internal validity.

Internal validity is a way of measuring if a research is sound or done the right way, it relate to how many confounding variable the researcher has in the experiment ( the less chance of confounding variable in that research, the higher its internal validity is). It reflect whether or not the research study will or are trustworthy and meaningful which help to validity how the study is conducted

d. the polar jet stream   lies between the polar easterlies and trade winds should be your answer